What does the stamp drop down menu in the render panel do?
The Stamp section in the Render panel goal is used to draw some information on the render output. This can be useful for a work in progress production.

See the blender wiki.

For example you can select Frame to print the current frame number on the final output.

The Font size, Text color and Background settings allow you to control how this information is displayed.

You can additionally print a custom note by checking the Note setting and writing whatever you want.